The present invention relates to a fuel supply control system for an internal combustion engine which is of the type having a fuel injection valve arranged in an intake manifold at a location upstream of a throttle valve for supplying fuel to all the cylinders and, more particularly, to a fuel supply control system of this kind which corrects an amount of fuel supplied to the engine, depending upon atmospheric pressure.
Conventionally, a fuel supply device for an internal combustion engine has been proposed, e.g., by U.S. Pat. No. 4,378,000, which engine has a fuel injection valve arranged in an intake manifold at a location upstream of a throttle valve for commonly supplying fuel to a plurality of cylinders thereof, thereby reducing the number of fuel injection valves employed and hence reducing the manufacturing cost of the fuel supply device. According to the proposed control device, an air throttle valve is arranged such that a notched opening formed therein is disposed opposite the nozzle of the fuel injection valve when it is closed, so as to increase the flow speed of intake air in the vicinity of the nozzle.
Further, a method has been proposed by Japanese Provisional Patent Publication (Kokai) No. 63-143346 by the present assignee, which controls an air throttle valve as mentioned above to assume its closed position when a predetermined low rotational-speed condition of the engine is satisfied, thereby improving the atomizing behavior of fuel injected into the intake manifold and and hence achieving stable driveability of the engine at low rotational-speed operation of same.
However, in the proposed method, a predetermined engine rotational speed for determining whether the predetermined low rotational-speed operating condition of the engine is satisfied or not is set at a fixed value and accordingly the timing, at which the air throttle valve is brought into its closed position with respect to the rotational speed of the engine, is not varied regardless of whether at high altitude or at low altitude the engine is operating. Therefore, when the engine is operating at high altitude where air has low density, the mass of intake air substantially decreases accordingly. As a result, the amount of intake air supplied to the engine becomes insufficient at acceleration of the engine immediately following engine operation under the predetermined low rotational-speed operating condition, whereby the engine cannot produce required output and hence has degraded driveability.